The 1972 horror film "Death Line" is an intriguing cut of cinematic meat. On one hand, it boasts a surprisingly lovable blue-collar police inspector character, played against type by Donald Pleasence. His Inspector Calhoun emerges as a special treat in a film that largely depends the promise of subterranean cannibal horrors in the London Underground.

But don't worry! There's horror aplenty in this film, perhaps more than most viewers bargain for. When not accompanying Inspector Calhoun on his drinking rounds, "Death Line" (or "Raw Meat" in the states) concerns a family of Tube-dwelling cannibals. Descended from Victorian railway workers buried alive during tunnel construction, the last of the subhumans embarks on a bleak, ultraviolet rampage.

Many horror fans hold "Death Line" up as a horror jewel, others find it hard to love such a deceptively vicious film. Wherever you fall on the spectrum, you might well wonder if such biological deviation is possible. Can a surface species, locked away in the train tunnels beneath London, transform into a true sub species?

For answers, we turn to the curious case of the London Underground mosquito...

Blood Drinkers in the Dark

First, let's refresh a bit regarding the Tube itself. Opening in 1863, it was the world's first underground railway. Like an expanding root system, it’s grown ever since. Today, 11 lines collectively handle approximately 4.8 million passengers a day through 270 stations and 250 miles (400 km) of track. A good 45 percent of that is underground.

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Cramped conditions, 1895.

Hulton Archive/Getty Images

The construction of those tunnels was a colossal undertaking. Laborers died and, yes, various organisms found themselves trapped within the confines of this new, artificial environment. We all know about the rats, but we can also count blood-sucking mosquitoes among the subworld inhabitants. Along with other parasites, the buzzing vamps made themselves known when Londoners flocked into the tunnels to escape enemy bombardments during the world wars.

Fifty years later, doctoral student Katharine Byrne went down to investigate. She collected mosquitoes from 20 London sites -- outdoor, indoor and subterranean locations. She reared them and attempted crossbreeding (between mosquitoes, not humans and mosquitoes). In 1999, she published her findings in the journal Heredity with fellow-researcher Richard A Nichol.

They found that Culex pipiens in the underground behaved rather differently from their surface world counterparts. Surface Culex pipiens only drink the blood of birds. But this new subspecies, Culex pipiens molestus, drank the blood of humans and rats.

It makes sense, right? There’s going to be plenty of it down there, taking into account all those commuters and Victorian cannibals.

Additionally, the Tube mosquitoes don’t require hibernation, mating swarms or blood to lay eggs. In other words, they're autogenous -- they don't need a blood meal to lay eggs. See, male and female mosquitoes consume nectar and plant juices. Blood-drinking females in many mosquito species just need a spot of gore to energize their egg production.

In effect, Culex pipiens adapted to the underworld. They became so distinct that they could no longer breed with their surface-dwelling kin. Different mating behavior. Different reproductive behavior. No mating swarms. Just individual lovers.

So the Tube-dwelling cannibals of the London Underground are not alone. Underground spaces and artificial environments have a way of changing us.

To quote "London: A Biography" author Peter Ackroyd, "The whole area under the river, and indeed under the whole of the city, is a catacomb of avenues and highways mimicking their counterparts above ground. Yet something happens when you travel beneath the surface of London; the very air itself seems to become old and sorrowful, with its inheritance of grief.”

Monster of the Week is a - you guessed it - regular look at the denizens is of our monster-haunted world. Sometimes we'll focus on the cultural aspects, but mostly we'll look at the possible science behind a creature of myth, movie or legend. Be sure to explore the Monster Gallery as well as the Monster Science video series.

About the Author: Robert Lamb spent his childhood reading books and staring into the woods — first in Newfoundland, Canada and then in rural Tennessee. There was also a long stretch in which he was terrified of alien abduction. He earned a degree in creative writing. He taught high school and then attended journalism school. He wrote for the smallest of small-town newspapers before finally becoming a full-time science writer and podcaster. He’s currently a senior writer at HowStuffWorks and has co-hosted the science podcast Stuff to Blow Your Mind since its inception in 2010. In his spare time, he enjoys traveling with his wife Bonnie, discussing dinosaurs with his son Bastian and crafting the occasional work of fiction.